1. Field of the Invention
This invention deals with well logging techniques, in particular with an improved dual spaced neutron well logging tool that is insensitive to borehole effects and insensitive to changes in the operational characteristics of the two neutron detectors.
2. Description of the Prior Art
For an evaluation of the commerical potential of an oil bearing formation, the porosity of the formation, as well as the oil saturation of the liquid filling these pores, must be known. Neutron well logging tools are suitable for the evaluation of the porosity as well as of the oil saturation, since they are strongly effected by the hydrogen content of the formation (which can be interpreted in terms of porosity) as well as to the salinity of the formation (which can be interpreted in terms of oil saturation).
The first commerical neutron logging tools simply comprised a single neutron source and a single neutron detector. The information derived by these tools was of rather limited value, since it did not separate the effects of formation porosity and formation salinity.
A second generation of neutron well logging tools utilized one source, two detectors and ratio building techniques to derive salinity independent porosity information. Such tools are very useful and widely applied at present. Although these tools substantially eliminate the salinity effects on the porosity reading, they still are responsive to borehole effects. Therefore, a third generation of neutron well logging tools was developed that contained means for the compensation of the borehole effect.
Several approaches were taken to eliminate or at least to reduce the borehole effect on the porosity reading of a neutron well logging tool. One simple way to achieve this is to design a decentralized tool with the source and detectors pressed against one wall of the borehole, as described in U.S. Pat. No. 3,521,062. On the other hand, more than two detectors can be used and the signal can be corrected for the borehole effect, as described in the U.S. Pat. No. 3,532,884 and 3,567,935.
U.S. Pat. No. 3,532,884 describes a pair of detectors used for the derivation of the ratio, and a third detector used for the correction of the ratio. U.S. Pat. No. 3,567,935 uses three or four detectors for the derivation of two ratios. The first ratio is the basic porosity indicator, whereas the second ratio is used to correct the first ratio.
Instrumentation other than neutron detectors can also be used for the correction of the neutron log. In U.S. Pat. No. 3,789,219, for example, a dual spaced gamma ray tool is used for the correction of the neutron log for the effects of the mudcake, and a borehole caliper is used for the correction of the neutron log for the effects of the borehole diameter.